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The orbit: Optic nerve complex:Optic nerve
Optic sheath * Dural encasement of nerve ** Contiguous with intracranial dura ** All 3 membrane layers of meninges present including pia, arachnoid & dura mater. The nerve is an extension of the brain. * CSF-filled arachnoid space surrounds nerve ** Contiguous with suprasellar cistern ** Transmits intracranial pressure changes The optic nerve is the second cranial nerve. It is covered by pia, arachnoid and dura mater. The optic nerve is divided into four segments # Intraocular segment: 1mm. within the retina and emerges through scleral opening (lamina cribrosa) from the convergence of the ganglionic cells. # Intraorbital segment: around 25 mm, which passes posteriorly and centrally within the intraconal orbital space. It is surrounded by three meningeal lining and CSF. The central artery and vein pierce the nerve about halfway. # Intracanalicular segment: 10mm, where the optic nerve exits through the tendinous ring and optic canal of sphenoid bone inferior to the ophthalmic artery with the nasociliary nerve and superior ophthalmic vein behind the artery. The ciliary ganglion lies on the lateral side of the optic nerve. The anterior part of the nerve is closely surrounded by the short ciliary nerves and vessels. # Intracranial or cisternal segment: which enters the middle cranial fossa, covered by pia and surrounded by CSF from suprasellar cistern. The anterior cerebral artery at its superolateral aspect. Joins the contralateral optic nerve at the optic chiasm. Optic chiasm * Horizontally oriented; "X-shaped" structure within suprasellar cistern * Forms part of floor of 3rd ventricle between optic recess anteriorly and infundibular recess posteriorly * Immediately anterior to infundibulum (pituitary stalk), superior to diaphragma sellae * In chiasm nerve fibers from the medial (nasal) half of retina cross to opposite side while the lateral (temporal fibres) do not. Optic tracts: From the optic chiasm arise two optic tracts each one contains nasal fibers of the contralateral optic nerve and temporal fibers from ipsilateral optic nerve. * Fibers pass posterolaterally curving around cerebral peduncle and divide into medial and lateral bands ** Lateral band (majority of fibers) ends in lateral geniculate body of the thalamus ** Medial band goes by medial geniculate body to pretectal nuclei deep to superior colliculi Optic radiation and visual cortex Arising from the lateral geniculate body of the thalamus, efferent axons form optic radiations (geniculocalcarine tracts) fan out from lateral geniculate body and run as broad fiber tract to calcarine fissure. * Initially pass laterally to the retrolenticular part of the internal capsule and basal ganglia * Extend posteriorly around lateral ventricle passing through posterior temporal and parietal lobes * Terminate in calcarine cortex (primary visual cortex) on medial surface of occipital lobes * Division: ** superior bundle carries information from the superior retinal quadrant that represents the inferior visual field and ends at the superior aspect of calcarine sulcus (cuneus). ** The inferior bundle (Meyer's loop) carries information from the inferior retinal quadrant which represents the superior visual field loops to inferior of the calcarine sulcus (lingual gyrus) Vascular supply * Ophthalmic artery ** First intradural branch of ICA ** Major arterial supply to orbit ** Passes through optic canal in dural sheath ** Exits sheath laterally at orbital apex * Central retinal artery ** 1st branch off the ophthalmic artery, supplies retina ** Enters optic nerve about 1 cm posterior to globe with accompanying vein to run to retina * Central retinal vein ** Accompanies central retinal artery ** Drains directly into cavernous sinus